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Using a large machine (4 E7-4860, 80 cores, 1TB memory), running ubuntu 14.04; virtual box 4.3.20 installed. Guest is win7 enterprise, allocated 16 cpus, 16GB ram and 256MB video ram; about 34GB free in a 64GB partition; has guest additions installed. Guest runs fine, it boots in a few minutes, it's responsive, etc.

Tried increasing the guest's virtual box setting for allocated processors from 16 to 32 (the max). Booted the guest to test. Result is that the guest slows way down - boot time increases by many minutes. When finally booted, it feels slow; mouse clicks don't result in fast responses, etc.

Restoring the setting to 16 processors brings back the acceptable boot time and performance.

Please advise, is 32 cores for a single win7 guest acceptable to virtual box? Am I doing something stupid? Thanks in advance.

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VirtualBox performance is terrible when you assign it more cores than are cores in a single CPU socket.

The most likely problem is that the guest OS see it as multiples cores in the same CPU (the guest sees a SMP system but the hardware is really NUMA) and creates lots of inter-CPU traffic.

Your setup should work well with 20 cores or less.

Source.

  • I should have said: the machine has 40 cores but is hyper threaded. I will go with 10 "Processors" in VirtualBox settings. Very useful, makes sense, ty. – chrisinmtown Jan 8 '15 at 15:19

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